1. Field of the Invention (Technical Field)
The present invention relates to the field of audio headsets, in particular to the field of mono and stereo headset interface with audio playback devices.
2. Background Art
Audio playback devices including radios, cassette tape players, compact disc players, digital audio tape players, and the like, are used to produce acoustic signals from prerecorded audio and broadcasted audio information. Depending upon the output configuration of the playback device and the recording format, audio information is output from the audio playback device in either mono or stereo format. Other audio formats are available but are not discussed here. A mono formatted recording provides audio information on a single mono channel, while a stereo formatted recording provides different audio information on each of multiple channels, typically left and right. The result of audio information recorded in a stereo format is that the user perceives a broader spaced sound that more accurately reproduces the sound from a live performance. Audio output from audio playback devices is sent to a speaker or speakers integral to the device, speakers remotely located from the device, or from speakers that are in close proximity to a listener's ears, commonly referred to as headsets or headphones.
When an audio playback device operating in stereo mode is connected to a stereo headset having left and right outputs, a stereo headset driver associated with the audio playback device provides the amplification necessary to drive the speakers in the headset. The headset is connected to a headset plug having electrically-isolated conductors for audio left, audio right, and ground, and a microphone input if the headset is equipped with a microphone for communication input to the playback device. A microphone input would be appropriate if the playback device is, for example, a mobile communication device, such as a cellular telephone. The headset plug fits into a mating headset jack located on the audio playback device. The headset jack interfaces with the audio left, audio right, ground and microphone inputs from the headset.
Typically a single power supply configuration supplies power to the headset driver of the audio playback device, resulting in a DC voltage level at the left and right outputs of the driver between ground and the single supply. For that reason, DC blocking capacitors are placed on the audio left and audio right signals between the headset driver and headset jack to filter the DC voltage from the signal.
When the stereo audio playback device is used to drive a mono headset, such as when a mono headset is plugged into the stereo headset jack of the playback device, the audio right signal is shorted to ground through the ground conductor of the mono headset plug. This occurs because the mono headset plug has just three conductors, ground, audio left, and microphone. When plugged into the stereo headset jack of the playback device, the microphone, ground and audio left interfaces of the headset jack align properly with those of the headset plug, but the audio right interface from the headset jack contacts the ground portion of the headset plug. Thus, when a mono headset plug is plugged into a stereo headset jack, excessive electrical current is drawn from the power supply by the headset driver because one of the audio outputs is grounded instead of being connected to the proper impedance load. This results in excessive heating of the headset driver and possible damage to the headset driver. For this reason audio playback devices that provide stereo output through a stereo jack are not compatible with standard mono headsets.
Devices used for wireless communication, including mobile cellular and satellite telephones, pagers, personal digital assistants (PDAs), and the like (hereafter “mobile communication devices”) conventionally provide mono playback of audio information. Headsets having a speaker sized for placement in the vicinity of the left or right ear of a user are often employed in conjunction with mobile communication devices to accommodate “hands-free” communication. The speaker is typically connected via a hard-wire connection to the mobile communication device through a headset jack on the device. As mobile technology advances, mobile communication devices will provide stereo audio output. A mobile configured to provide stereo audio output will not be compatible with existing standard mono headsets due to the incompatibility of mono headset plugs to stereo headset jacks.
It would be advantageous if a headset interface of an audio playback device that outputs stereo formatted audio information was compatible with both stereo and mono headsets. Such an interface would provide the user the capability to use standard mono headsets with audio playback devices that output stereo audio.